How to Have Selected Faces Emit Smoke
Anand mbs is correct about selecting faces, but to use those selected faces in a Smoke Simulation, you need to do some more stuff.

Selecting The Faces

At the bottom of the 3D Viewport and while in Edit Mode, you can click a button that looks the following to switch into Face Select mode.



However, also while in Edit Mode, you can key Ctrl + Tab to open a pop-up menu like the following. You can then either click the bottom option (Face) or key 3 to have Blender automatically select the third option down.



Separating Faces

We're not quite done yet. RMB select the first face and then Shift + RMB select the rest that you want smoke to emit from. Key Shift + D to duplicate the faces. Key P to bring up the SeParation pop-up menu. It looks like the following. Key 1 or click on Selection to separate the faces from the original mesh.



Smoke Simulation

Tab back into Object Mode. Select the mesh that is made up of the separated faces. This will be your Inflow object. It is also probably wise to Parent the faces mesh to the original. Parenting will guarantee that the faces mesh follows the original during transforms. Select the faces mesh, then the original, then key Ctrl + P and either 2 or select Object (Keep Transform) from the resulting pop-up menu.